In telecommunication and radio communication, data can be represented by narrow frequency band signals generated using shift keying techniques. There are different forms of shift keying those related to amplitude shift keying (ASK) or frequency shift keying (FSK) and those related to phase shift keying (PSK) such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK) and offset quadrature phase shift keying (O-QPSK).
In order to achieve resistance to natural interference, noise and jamming, to prevent detection, and to limit power flux density the resulting narrow band signal is not transmitted as such but spread over a larger or wider frequency band.
Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence, frequency hopping, or a combination of both.
Spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a wider band of frequencies (wideband radio). The receiver correlates the received signals to retrieve the original information signal.
Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. Each of these techniques employs pseudorandom number sequences—created using pseudorandom number generators—to determine and control the spreading pattern of the signal across the allocated bandwidth.
DSSS uses a signal structure in which the sequence of chips produced by the transmitter is already known by the receiver. The receiver can then use the same pseudo noise code symbol sequence to counteract the effect of the pseudo noise code symbol sequence on the received signal in order to reconstruct the information signal. DSSS phase-modulates a sine wave pseudo randomly with a continuous string of pseudo noise code symbols called “chips”, each of which having a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
Another standard, IEEE 802.15.4-2006, covers several physical layers, using several modulation techniques, operating in wide range of the frequencies, where three major frequency bands are utilized, i.e. sub-GHz (between: 314 MHz and 956 MHz), 2.45 GHz ISM Band (between 2400 MHz and 2483.5 MHz), and ultra wide band (UWB) only: below 1 GHz, between 3 GHz and 5 GHz and between 6 GHz and 10 GHz. In Ultra-wideband (UWB) modulation is commonly based on transmitting short duration pulses. Wireless Ethernet standard IEEE 802.11 uses either FHSS or DSSS in its radio interface.
One of the most interesting sub-GHz Bands is called “g1” Band, covering frequencies between 868.0 MHz and 868.6 MHz. Frequency bandwidth is narrow—only 600 kHz—preventing high data rates in wireless communication where simple modulation schemes are used.
According to the IEEE standard 802.15.4-2006, 250 kbps is the maximum possible gross data rate specified for the 868.3 MHz band of 600 kHz frequency bandwidth, the “g1” band. But due to the narrow frequency bandwidth, prior art implementations exhibit in practice much lower values of the gross data rate—in order of 100 kbps, maximally.
Tsai Y., Mary spreading-code-phase-shift-keying modulation for DSSS multiple access systems, IEEE Transactions on Communication, Volume 57, Issue: 11, pages 3220-3224, (November 2009), describes that code shift keying (CSK) was proposed to increase the transmission efficiency of DSSS systems, and to overcome the spreading gain versus data rate limitation and proposes to improve the system flexibility by switching the spreading code phase in accordance with the incoming data.